Methods of and apparatus for generating seismic signals

ABSTRACT

Seismic waves are produced at a high rate by a method and apparatus which utilise the successive detonation comprising fragmented explosives. Wads of an explosive from 1 to 30 percent as much of a substance intended to lower the electrical resistance of the explosive as there is of said explosive, and from 1 to 30 percent as much of an oxidisable substance as there is of said explosive are propelled, by a compressed gas, along a duct opening into the outside medium. A high voltage is applied between an electrode at the outlet of the duct and an electrode spaced from this outlet, which voltage is applied to the electrodes at an instant determined by the passage of the wad of explosive near the duct outlet.

United States Patent 1191 Barbier et al. Apr. 9, 1974 METHODS OF AND APPARATUS FOR 3.368.641 2/1968 ChOltil 181/.5 NC GENERATING SEISMIC SIGNALS 2,770,l93 11/1956 Eisler 181/15 NC 3,064,753 11/1962 McClure l8l/.5 NC [75] lnventors: Maurice Bar-bier, Ousse; Sayous Leon both of France Primary Examiner-Benjamin A. Borchelt [73] Assignee: Societe Anonyme dite: Societe Assistant Doramu's Nationale Des Petroles DAquitaine, Atlfomey, Agent, FirmBri5ebi5 & Kruger Courbevoie, France 221 Filed: Aug. 24, 1972 7 Z Z h b e1sm1c 'waves are pro uce at a v1g rate y a [21 1 App! 283268 method and apparatus which utilise the successive det- R l t d U5, A li ti D t onation comprising fragmented explosives. Wads of an [63] Continuation of Ser No n88, Feb 10 1970 explosive from 1 to 30 percent as much ofa substance abandoned intended to lower the electrical resistance of the ex- [30 1 Foreign Application p i Data plosive as there is of said errplosive, and from l,to 3 0 F b 1969 F t 69 03,76 percent as much of an ox1d1sable substance as there 15 e rance of sand exploslve are propelled, by a compressed gas, [52] U.S. Cl. 181/5 XC along a duct opening i the outside i A high [5|] Int. Cl.. GOlv 1/12, GOlv 1/06 voltage i applied between an electrode at the Outlet [58] Field of Search 181/5 XC! A; of the duct and an electrode spaced from this outlet, 340/3 T which voltage is applied to the electrodes at an instant determined by the passage of the wad of explosive [56] References Cited near the duct outlet.

UNITED STATES PATENTS 6 Claims, 4 Drawing Figures I 3,382,946 5/l968 Smith 181/.5 NC 3,263,608 8/1966 Andrews 181/.5 NC

PATENTEDAPR 91974 3,802,534

sum 1 [IF 2 I A a 12 E METHODS OF AND APPARATUS FOR GENERATING SEISMIC SIGNALS This is a continuation, of application Ser. No. 10,188, filed Feb. 10, 1970, now abandoned.

The present invention relates to a method of generating seismic signals in the form of pulses at a high rate, which it does by utilising the successive detonation of fragmented explosives, and to apparatus for doing this.

On the one hand, it is known to generate seismic signals with the aid of explsove charges which are placed in the ground or in the sea at a certain depth. These explosives are fired by primary detonators, which may be operated by, for example, an electric current.

Conductive explosives are also known, in which an oxidisable metallic charge and a material which is intended to render the explosive electrically conductive are mixed with a secondary explosive. These explosives may be fired by the action of an electric discharge of sufficient power, for example in the space between two electrodes.

Discontinuous seismic transmission with the aid of controlled electric spark gaps is also known. Such sources permit the transmission of seismic signals into the ground at short intervals. However, the total power transmitted by these sources remains low. It is thus necessary either to use a source having a high momentary power, but the average power of this is determined by the power supplied by the generator, which results in a transmission signal pulse at a very low rate; alternatively, if transmission at a high rate is required, the momentary power of each pulse is reduced. The depth of exploration using such sources is therefore small. They cannot be used to resolve geophysical problems at great depths, which are the most important ones.

The present invention sets out to resolve or reduce these problems and to make it possible to explore depths, which may be comparable to the depths which can be reached by the use of methods employing heavy-charge explosives, in which an explosive charge is repeatedly exploded between two electrodes, to which there are applied voltages whose vairation is related to the presence of explosives between the electrodes.

The method of producing seismic signals according to the present invention comprises disposing in a supply container a homogeneous mixture comprising a secondary explosive, from 1 to 30 percent as much of a substance which lowers its electrical resistance as there is of said explosive and from 1 to 30 percent as much of an oxidisable substance as there is said explosive, causing the mixture to flow through a duct, sequentially injecting into this duct a pulsed pressure of compressed gas so as to divide the mixture in the duct into sections and to cuase these sections downstream of the duct to travel at an increased speed so that the sections are brought in succession into a space between two electrodes. detecting the passage of each section as it is thus propelled by the gas, producing, as a result of this detection and after a time delay, a high voltage between the end of the duct and an electrode situated at an appropriate distance from the latter, so that the presence of the explosive mixture and the application of the voltage are synchronised, the said voltage then detonating the explosive present in the space between the two electrodes.

In a preferred embodiment of the invention, an annu- I lar stream of water is passed around the stream of mixture leaving the duct; this is intended to canalise the stream of mixture in such a manner as to give it the form of an elongate cylinder.

In accordance with a further feature of the invention, the gas is air and it is injected into the stream of mixture at predetermined intervals and for predetermined times.

IN accordance with a further feature of the invention, the air injection intervals and the duration of these injections are determined with the aid of a graphic or other suitable record.

In accordance with yet another feature of the invention, the instant at which the voltage is applied between the two electrodes is determinedby means of a device for detecting the presence of the mixture, which device supplies an electrical signal, which is' delayed and utilised for controlling the circuit through which the high voltage is applied to the electrodes.

The invention also concerns apparatus for carrying out the above method. Such apparatus comprises areservoir adapted to contain an explosive mixture as defined above, a constant-delivery pump, a duct into which this pump delivers the mixture from the reservoir, a compressed-gas supply conduit leading into the duct downstream of th pump, a device having detecting means adjacent th duct for detecting the presence of the mixture, the said detecting device supplying an electric signal, a high-voltage discharge source which is adapted to be connected to a high voltage generator and which is itself connected, on the one hand, to the duct and on the other hand to an electrode spaced from the duct, the high voltage discharge source comprising a discharge control system which is operated by detecting device, and a valve which is arranged in the compressed-gas supply conduit to modulate the passage of the compressed gas, which it preferably does in accordance with a time code suppied by a graph-reading member.

The invention will be more readily understood from the following description of a form of construction of the apparatus and of a mode of application of the method, the description being illustrated by the accom panying drawings.

In the drawings:-

FIG. 1 shows a formof construction of apparatus exemplifying the invention;

FIG. 2 illustrates the explosive-mixture outlet of the apparatus shown in FIG. 1, at the moment preceding the firing of the explosive;

FIG. 3 illustrates graphically the various voltages which are set up in the apparatus; and

FIG. 4 illustrates the pressure waves which result from the firing of the explosive.

In FIG. 1, there is illustrated a mixing and kneading hopper l which serves to store the explosive mixture. This is fed by gravity to a constant-delivery pump 2, which delivers it into a duct 3 of generally cylindrical form.

Leading into the duct 3 is a conduit 4 supplying the compressed air which is provided by a compressor (not shown). Connected to this conduit 4 is a valve 5 which is operated by a coil 6. This coil is controlled by the arm 7 of a relay 8, which relay is connected to a voltage source 9, the outut of which is controlled by graphs plotted on a tape 10, which is read by a reading head l l, the said tape being driven at a constant speed by a motor 12.

The relay 8 controls, at the same time as it controls the valve 5, the electrical supply to the pump 2 through cables 13. The supply to the pump 2 is complementary to the closing of the electromagnetic valve 5; that is to say, the pump 2 operates when the valve 5 is closed and is stopped when the valve 5 is open.

A continuously operating pump 14 injects water into an annular passage 15 surrounding the duct 3. The resulting water stream has the form of an annular cylinder which escapes at the annular end 16 of the passage 15. A detector 16, for example one of the resistance threahold type, has two electrodes 18 and 19 situated in the duct 3. This threshold detector supplies an electric signal when the resistance bewtween 18 an 19 passes a given limit.

This electric signal is delayed in a monostable trigger circuit 20 and is then amplified of a power generator 21, which is controlled by the signal emitted by the circuit 20, and the resultant signal is applied to the central electrode 22 of a discharge control triggering device 23. This triggering device 23 controls the discharge of a capacitor 24, which is supplied by a transformer 25 followed by two recitifiers 26a and 26b. The primary winding of the transformer 25 is supplied from a alternating-voltage source 27. An electrode 28,which is connected to the capacitor 24 through the triggering deivce 23, receives the high voltage which is supplied by the capacitor when 23 is conductive.

This apparatus operates as follows:

There is introduced into the hopper 1 a mixture of explosive, oxidisable powder and a material intended to reduce the electrical resistance of the mixture.

For example, there may be employed as the explosive a mixture of pentrite and ammonium nitrate in powder form, to which there are added graphite, naphthalene or one or more metallic sulphides; the oxidisable substance might be either magnesium or aluminium in powder form. The mixing and kneading hopper renders the mixture permanently homogeneous and the pump 2 injects this mixture at a constant rate into the duct 3. The mixture continues as far as the junction of the duct 3 and conduit 4.

It will be assumed that the pump 2 is delivering the mixture and that the valve 5 is closed. The mixture passes beyond the junction of the ducts 3 and 4. An order coming from the device 11 triggers 9; valve 5 then opens, and the pump stops.

In one construction, the pump 2 continues to operate and delivers into a by-pass, which opens at the same time as the electromagnetic valve 5. This by-pass is not shown in the drawings.

That fraction of the mixture which has proceeded beyond the junection of the ducts 3 and 4 is separated into sections or fractionated by the compressed air and these are propelled at high speed down the interior of the duct 3. Very shortly after it has left the junction each section passes the electrodes 18 and 19, which are so positioned that the distance between them corresponds substantially to the length of the wad of explosive mixture which it is desired to use. As a result of the passage of the wad, a change of resistance is detected by the device 17 and this sets up, through 20 and 21, a voltage at the electrode 22, which controls the discharge triggering device 23.

The capacitors 24, to which a high voltage was previously applied by the charging device 25, 26, 27, supply a high voltage which is applied between the electrode 16, consisting of the concentric ends of the pipes 3 and 15, and the positive electrode 28. At this precise instant, the explosive mixture is also located between these two electrodes, where it is surrounded by a sheath of water which has been projected through the passage 15.

The high voltage passes through the explosive and causes the latter to explode.

The explosive is fired and produces a mechanical force wave which is propagated through the surrounding medium. The explosive is limited to the quantity of explosive mixture present between the electrodes, the prepelling air stopping the propagation of the explosion towards the actual apparatus.

At the instant when the explosion takes place, the electromagnetic valve 5 is again closed, the relay 6 not being energised. The pump 2 again supplies a certain part of the explosive mixture, which travels beyond the junction of the ducts 3 and 4. Some time after this, the order to open the electromagnetic valve 5 is again given by the unit 9 a further quantity of explosive is forced by the compressed air into the region between the electrodes 16 and 28. An explosion is again produced, and the cycle recommences. It is thus possible to produce a series of successive explosions, each explosion being limited to the zone situated between the electrodes 16 and 28 owing to the presence of air and water in the ducts 3 and 4 and passage 15, so that only the desired quantity can be fired.

Many kinds of explosives are suitable for this apparatus. More particularly, there may be employed, in admixture with a basic explosive (pentrite or tolite), an exothermically decomposing body such as a glycol, nitrate or chlorate.

FIG. 2 illustrates the position of the explosive mixture at the instant preceding the firing.

The references are similar to those in FIG. 1, thus 18 and 19 indicate the electrodes for the measurement of the resistance change and 17 indicates the device which supplies a pulse as a function of the resistance change measured between the electrodes 18 and 19. The duct 3 for the supply of the explosive mixture is surrounded by the water supply duct 15. These ducts lead into the outside medium through two concentric orifices, shown at 16. There is also shown a protective hood 29, which is intended to prevent the propagation of the explosive wave back towards the apparatus.

The explosive section is shown at 30. It is localised between the point of its discharge 16, which serves as a negative electrode, and the electrode 28, which is positively changed by the high voltage. There is shown at 31 the stream of water which forms a kind of sheath for the explosive mixture. Under the action of the high voltage applied between the electrodes 16 and 28, the wad 30 of explosive mixture explodes and the resultant gases are dispersed so as to form bubbles in the ambient medium.

The air which has been employed to discharge the wad 30 enters the bubbles containing the explosive gas. The stream of water 31 fills the centre of the bubble, which is thus moved clear of the explosion zone, assisted by the fact that the apparatus itself will be in motion through the water. This is the case in marine seismology in which the apparatus will move about 1 metre between two explosions separated by an interval of 200 milliseconds.

Some time after an explosion, a further wad of explosive mixture will pass through the duct 3 and replace the wad shown at 30. The explosion of this fresh wad is effected by the application of the high voltage between the electrodes 16 and 28 (this application coinciding with the arrival of the second wad of explosive mixture at this point) and the cycle can thus be repeated indefinitely. The stream of water coming from the duct permits of an almost immediate filling of the central part of the bubble, thus recreating a continuous medium through which a further mechanical wave will be transmitted to the ambient medium. The duration of the explosion is fixed, on the one hand, by the volume of the wad 30 and, on the other hand, by the voltage applied to the electrodes 16 and 28 and by the power dissipated in the are formed in the mass of explosive mixture. The interval between two wads of explosive mixture is adjusted by the pattern of the control graphs, as read by the reading head 11 shown in FIG. 1.

There are shown in FIG. 3 the forms of the various voltages as plotted at various points in the apparatus. Thus 32 indicates two square waves corresponding to the voltage supplied by the reading head 11, and 33 shows two voltages set up across the terminals of the control coil 6 of the electromagnetic valve 5. These last two voltages are coincidental to within the response time of the relay 8.

The line 34 is the supply voltage of the pump 2. This voltage may be independent of the supply voltage of the relay 33. It may also be produced in a complementary manner. The duration of the operation of the pump 2 controls the quantity of explosive which will be detached by the stream of compressed air and which will explode between the electrodes 16 and 28, since the volume of each fraction of explosive mixture is the product of the cross-section of the duct 3 multiplied by the length beyond the junction of the ducts 3 and 4. It is possible to use a fixed operation time so as to obtain a constant quantity of explosive each time. It is also possible, by modifying the duration of the operation of the pump 2, to obtain a variable quantity of explosive within the limits defined by the space between the electrodes 16 and 28.

The curve 35 shows the pulses emanating from the threshold triggering device 17. The fact that a resistance which is higher or lower than a predetermined value is detected between the measuring electrodes 18 and 19 causes th triggering device to change over, and the latter thus supplies a voltage peak 35. The curve 36 shows this voltage peak retarded through the monostable trigger circuit 20, after amplification at 21. This voltage peak is applied to the electrode 22 controlling the high-voltage discharge circuit. The curve 37 represents the high voltage which is supplied in the interelectrode space 16-28 and which serves to fire the explosive. This voltage is of the-order of 10,000 volts and the inter-electrode capacitance may be 2 microfarads. The pressure wave is therefore set up substantially at the instant of application of the high voltage and it lasts during the rapid combustion of the explosive. It will thus be seen that there is a constant interval between the application of the voltage to the electromagnet 5 and the commencement of the explosion. On the other hand, the interval of time between the beginning of the two square waves 32 may be adjusted 'as desired, that is to say, the explosion rate may be adjusted as desired, as a function of the programme recorded on the tape 10.

It is therefore possible by this method to produce, at adjustable rates, explosions of predetermined unit energy. This may be employed with advantage, instead of the conventional spark sources or gas guns, in that it may multiply the usable energy by factors ranging from 5 to 100, while controlling the quantity of explosive per unit explosion and the repetition rate of each of these explosions.

FIG. 4 illustrates, by means of the curve 38, an explosion cycle as determined by a graph on the tape 10. Various pressure waves are shown, the durations of which are indicated at 39, 41, 43, 45, 47, 49; they are separated by intervals 40, 42, 46, 48.

While the durations 39, 41, 43, 45, 47 and 49 are substantially constant, the intervals 40, 42, 46 and 48 may be adjusted substantially as desired; There is a lower limit to these intervals, which corresponds to the time taken for the conveyance of the explosive mixture by the compressed gas through the duct 3. On the other hand, there is no upper limit to these intervals. I

The intervals between explosions may be so programmed that the time distribution of the mechanical 1. A method of generating seismic signals in the form of rapidly repeated pulses, utilizing the successive detonation of divided sections of an explosive, which method comprises introducing into a supply container an explosive mixture comprising a secondary explosive, a substance which reduces the electrical resistance of said secondary explosive, and an oxidisable substance, the quantity of each of said substances being equal to form 1 to 30 percent of the quantity of said secondary explosive, discharging the explosive mixture from said container and causing it to flow through a duct, sequentially injecting into the said duct a series of pulses of a compressed gas which divice the explosive mixture into sections and causes the downstream sections of the mixture to travel down the duct at an increased speed so that they are discharged at intervals of time into a space between the end of said duct, which serves as a first electrode, and a second electrode which is spaced from the first electrode in the zone which receives the stream of gas leaving the duct, detecting the passage of the sections of explosive as they are propelled by the gas, applying, as a result of this detection, a high voltage between the end of said duct and said second electrode, with a time delay such that the presence of the explosive mixture between the electrodes is synchronized with the application of the high voltage at the electrodes, the said voltage having the effect of electrically detonating each section of the explosive individually while it is in the space between the two electrodes, forming around said duct an annular stream of water which travels in the same direction as said explosive mixture and is ejected around the explosive mixture leaving the duct to form a sheath which maintains said explosive mixture in an elongate, generally cylindricalform.

2. A method according to claim 1, wherein the gas is injected into the stream of explosive mixture in said duct at predetermined intervals for predetermined periods of time.

3. A method according to claim 2, wherein the gas injection intervals and the duration of these injections are controlled by means including a graphic control record.

4. A method according to claim 1, wherein the instant at which the said high voltage is applied between said two electrodes is determined by means comprising a device for detecting the presence of the explosive mixture and supplying an electric signal indicative of said presence which signal, after having been delayed in time, is used to control the circuit through which said high voltage is applied to the electrodes.

5. Apparatus for generating seismic signals in the form of rapidly repeated pulses utilizing the successive detonation of divided sections of an explosive mixture, said apparatus comprising a reservoir for the said mixture, a constant-delivery pump, a duct into which the said pump delivers the mixture from said reservoir, a

compressed-gas supply conduit leading into the duct downstream of said pump, a valve which is arranged in the compressed-gas supply conduit to modulate the passage of the compressed gas and thereby divide said mixture into sections, a device for detecting the presence of each section of the mixture at points in said duct and supplying an electric signal, and a highvoltage discharge system which is adapted to be connected to a high voltage generator and which is itself connected, on the one hand, to said duct and on the other hand, to an electrode spaced from said duct so as to produce a high voltage discharge therebetween, said high-voltage discharge system comprising discharge control means which is operated by the said detecting device each time a section is detected, and means for forming an annular stream of water encircling said duct and projecting said stream around said mixture in the same direction as said mixture as it leaves said duct.

6. Apparatus according to claim 5, comprising means for reading a pre-recorded time code which controls the valve in the compressed gas supply conduit. 

1. A method of generating seismic signals in the form of rapidly repeated pulses, utilizing the successive detonation of diviDed sections of an explosive, which method comprises introducing into a supply container an explosive mixture comprising a secondary explosive, a substance which reduces the electrical resistance of said secondary explosive, and an oxidisable substance, the quantity of each of said substances being equal to from 1 to 30 percent of the quantity of said secondary explosive, discharging the explosive mixture from said container and causing it to flow through a duct, sequentially injecting into the said duct a series of pulses of a compressed gas which divide the explosive mixture into sections and causes the downstream sections of the mixture to travel down the duct at an increased speed so that they are discharged at intervals of time into a space between the end of said duct, which serves as a first electrode, and a second electrode which is spaced from the first electrode in the zone which receives the stream of gas leaving the duct, detecting the passage of the sections of explosive as they are propelled by the gas, applying, as a result of this detection, a high voltage between the end of said duct and said second electrode, with a time delay such that the presence of the explosive mixture between the electrodes is synchronized with the application of the high voltage at the electrodes, the said voltage having the effect of electrically detonating each section of the explosive individually while it is in the space between the two electrodes, forming around said duct an annular stream of water which travels in the same direction as said explosive mixture and is ejected around the explosive mixture leaving the duct to form a sheath which maintains said explosive mixture in an elongate, generally cylindrical form.
 2. A method according to claim 1, wherein the gas is injected into the stream of explosive mixture in said duct at predetermined intervals for predetermined periods of time.
 3. A method according to claim 2, wherein the gas injection intervals and the duration of these injections are controlled by means including a graphic control record.
 4. A method according to claim 1, wherein the instant at which the said high voltage is applied between said two electrodes is determined by means comprising a device for detecting the presence of the explosive mixture and supplying an electric signal indicative of said presence which signal, after having been delayed in time, is used to control the circuit through which said high voltage is applied to the electrodes.
 5. Apparatus for generating seismic signals in the form of rapidly repeated pulses utilizing the successive detonation of divided sections of an explosive mixture, said apparatus comprising a reservoir for the said mixture, a constant-delivery pump, a duct into which the said pump delivers the mixture from said reservoir, a compressed-gas supply conduit leading into the duct downstream of said pump, a valve which is arranged in the compressed-gas supply conduit to modulate the passage of the compressed gas and thereby divide said mixture into sections, a device for detecting the presence of each section of the mixture at points in said duct and supplying an electric signal, and a high-voltage discharge system which is adapted to be connected to a high voltage generator and which is itself connected, on the one hand, to said duct and on the other hand, to an electrode spaced from said duct so as to produce a high voltage discharge therebetween, said high-voltage discharge system comprising discharge control means which is operated by the said detecting device each time a section is detected, and means for forming an annular stream of water encircling said duct and projecting said stream around said mixture in the same direction as said mixture as it leaves said duct.
 6. Apparatus according to claim 5, comprising means for reading a pre-recorded time code which controls the valve in the compressed gas supply conduit. 